Antennas are conductors placed in space to radiate radio waves or induce electromagnetic force effectively in the space for communication, or devices for receiving and transmitting electromagnetic waves.
Antennas have common basic principles, but the shapes of antennas vary with the frequency used, to which the antennas are made to resonate for effective operation.
However, for there are various radio communication standards, which use different frequencies to one another, an antenna must have a plurality of resonant frequencies to be used for all standard. Further, recently portable radio communication devices have integrated functions including GPS, data communication, authentication, e-payment, etc. as well as voice communication, expanding application thereof, and these functions use different frequency bands, increasing the need for multi-band antenna.
For example, there exists the need for operating one radio communication device at 800 MHz band for DCN (Digital Cellular Network), GSM850 and GSM900, 1800 MHz band for K-PCS, DCS-1800 and USPCS, 2 GHz for UMTS, 2.4 GHz for WLL, WLAN and Bluetooth, and 2.6 GHz for satellite DMB, growing necessity of developing multi-band antennas.
In the meantime, radio communication device, which has become a necessity in modern life, tends to be smaller and lighter and so does antenna. Therefore, in these days antenna developers are in a technical and strategic position where they have to develop smaller but high-performance antennas.
Especially, recently the design of mobile radio communication device become various and built-in antennas that allow for high degree of freedom without affecting appearance of the device are employed much more than the past. In accordance, the main task in antenna research and development is to implement multi-band antenna having a plurality of resonant frequencies in limited and narrow interior space of communication devices effectively.
For convenience, conventional multi-band antennas are shown in FIGS. 1, 3 and 5.
FIG. 1 shows a conventional triple-band antenna. The antenna comprises ground plane 60, feed part 40, ground part 50, and the first to third radiating elements 10, 20 and 30. The conventional antenna exhibits triple band resonance characteristic, as shown in FIG. 2. In other words, the antenna of FIG. 1 has three resonant frequencies including the first resonant frequency around 800 MHz, the second resonant frequency around 1.8 GHz, and the third resonant frequency around 2.4 GHz. These resonant frequencies are determined by electrical lengths of the first radiating element 10, the second radiating element 20 and the third radiating element 30, respectively.
As shown in FIG. 3, if the second radiating element 20 is removed from the triple-band antenna of FIG. 1, it exhibits a resonant characteristic totally different from what it showed before with the third resonant frequency moved toward 1.8 GHz band as shown in FIG. 4.
Similarly, if the third radiating element 30 is removed from the conventional triple-band antenna as shown in FIG. 5, the first resonant frequency moves toward high frequency region, thus the frequency characteristics around the second resonant frequency is also changed drastically.
Generally, because for multi-band antennas, radiating elements should be placed in narrow and limited space achieving multi-resonant characteristics, radiating elements with various lengths, widths, and shapes are employed. In this case, as above, when adjusting one of resonant frequencies, another resonant frequency is changed due to the undesired inter-element effect.
Therefore, to set desired multi-band resonant frequencies, one resonant frequency is adjusted first, another frequency is adjusted, and finally, the resonant frequency adjusted previously has to be re-adjusted finely. Accordingly, as the number of radiating elements, thus number of frequency bands increases, the number of steps needed to adjust resonant frequencies increases exponentially and too much time and effort are required to develop an antenna.